Efficient Polarization Independent Single Photon Detector

ABSTRACT

A superconducting nanowire single photon detector (SN-SPD) microelectronic circuit is described which has higher quantum efficiency and signal-to-noise than any SN-SPD&#39;s known in the art. The material and configuration of the microelectronic circuit eliminates the polarization dependence and shows improved signal-to-noise over SN-SPD microelectronic circuits known in the art. The higher efficiency, polarization independence, and high signal-to-noise is achieved by vertically stacking two tungsten-silicide (TS) SN-SPDs and electrically connecting them in parallel. This structure forms a multilayer superconducting nanowire avalanche photo-detector (SNAP). A single photon detection device employing the multilayer (SNAP) microelectronic circuit demonstrates a peak system detection efficiency of 87.7% and a polarization dependence of less than 2%. This represents nearly an order of magnitude improvement in both system detection efficiency and reduction of polarization dependence compared to conventional SNSPDs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/638,675 filed on Apr. 26, 2012.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

FIELD OF THE INVENTION

The present invention relates to the field of single photon detectors,and more specifically to a photon detector which more efficientlydetects photons in low light environments.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a cut-away perspective of an exemplary single layersuperconductive nanowire meander pattern using a tungsten silicide (TS)nanowire.

FIG. 2 is a top view of an exemplary single layer superconductivenanowire illustrating a meander pattern using a tungsten silicide (TS)nanowire.

FIG. 3 is a side sectional perspective view of an exemplary embodimentof a multilayer polarization independent superconducting nanowireavalanche photo-detector (SNAP) microelectronic circuit.

FIG. 4 a is a scanning electron micrograph of an exemplary embodiment ofa multilayer SNAP microelectronic circuit.

FIG. 4 b is a close-up scanning electron micrograph of an exemplaryembodiment of a multilayer SNAP microelectronic circuit.

FIG. 5 is a wiring diagram of how an exemplary embodiment of amultilayer SNAP microelectronic circuit is wired.

FIG. 6 a is a top perspective view of an exemplary embodiment of amultilayer SNAP device.

FIG. 6 b is a diagram of the optical components of an exemplaryembodiment of a multilayer SNAP test system.

FIG. 7 a is a plot showing the system detection efficiency (SDE) of anexemplary embodiment of a multilayer SNAP device as a function of biascurrent.

FIG. 7 b is a plot showing the dark count rate (OCR) of an exemplaryembodiment of a multilayer SNAP device as a function of bias current.

FIG. 8 a is a map of the single-photon detection efficiency (SDE) overthe Poincaré sphere of an exemplary embodiment of a multilayer SNAPdevice.

FIG. 8 b is a map of the single-photon detection efficiency (SDE) overthe Poincaré sphere of a standard superconducting nanowire single photondetector (SN-SPD) device.

FIG. 9 a is a plot of the maximum and minimum SDE over the Poincarésphere of an exemplary embodiment of a multilayer SNAP device.

FIG. 9 b is a plot of the ratio of the maximum to minimum SDE(SDE_(max)/SDE_(min)) as a function of wavelength for a InGaAs powermeter and for an exemplary embodiment of a multilayer SNAP device.

TERMS OF ART

As used herein the term “nonmetallic material” refers to a materialwhich is not metallic. The term “non-metallic materials” includesmaterials such as silicon known as metalloids.

As used herein the term “continuous meander pattern” refers to amicroelectronic circuit characterized by a series parallel circuit linesjoined by curved segments to form a continuous electrically continuoustwisting and turning pattern made by a nanowire.

As used herein the term “patterned” refers to the attachment anddefinition of a layer of material in the form of a specified pattern ona base material.

As used herein the term “electrically connected” refers to componentsconnected in such a way that electrons can flow between them.

As used herein the terms “orthogonal” or “orthogonally” refers to therelationship of two intersecting lines in a common plane or therelationship of two lines in different planes which would intersect eachother if superimposed in a common plane. For example, orthogonallyrefers to the relationship of two meander patterns having segments whichare perpendicular or substantially perpendicular to each other.

As used herein, the term “reactive surface” superconductive nanowiresingle photon detector microelectronic circuit means the combinedsurface of the continuous meander pattern of superconductive nanowiresavailable to photons on the surface of the microelectronic circuit.

As used herein, the term “SNAP” means a superconducting nanowireavalanche photo-detector.

As used herein the term “tungsten-silicon alloy” or “tungsten silicide(TS)” means an amorphous material.

As used herein, the term “detection area” refers to the area on a photodetection microcircuit device which is capable of detecting photons.

BACKGROUND

Measurement of the timing and characteristics of photonic emissions iscritical to many scientific applications. Photons are characterized bytheir wavelength, their polarization, and by their location in time andspace.

A single photon detector (SPD) is a device which produces an electricalsignal when a single photon is absorbed by the detector. SPD's include asingle photon detector component which absorbs the photon and undergoesa change in state which produces the electrical signal. Recently singlephoton detector components have taken the form of solid statemicroelectronic circuits made using fabrication methods known in the artof solid state microelectronics. SPD devices also include opticalcomponents necessary to direct the light to the SPD microelectroniccircuit and electrical components to amplify and process the electricalsignals produced by the microelectronic circuit.

SPD's are widely used in scientific research in the fields of medicine,biology, astronomy, physics, chemistry, electrical and chemicalengineering, material science, and aeronautics. Additionally,single-photon detectors are an essential tool for a wide range ofapplications in quantum information, quantum communications, and quantumoptics.

The effectiveness of a photon detection device is measured in termsefficiency. Ideally the detector should produce a signal every time asingle photon enters the device. The probability that an electricalsignal will be produced when a photon enters the SPD (expressed as apercent) is referred to as the system efficiency. The probability that aphoton contacting the SPD microelectronic circuit will produce anelectrical signal is known as the quantum efficiency of themicroelectronic circuit. The wavelength, polarization, and the positionof the light all affect the system efficiency of the SPD device, andquantum efficiency of the microelectronic circuit. The system efficiencycan be no greater, and is frequently less, than the quantum efficiencyof the microelectronic circuit.

Other characteristics of SPD's used to determine their suitability forcertain applications are: signal-to-noise ratio, timing jitter, resettimes, and dark count rate. Signal to noise ratio is the ratio ofmagnitude of the electrical signal produced from the device to themagnitude of electrical noise of the device. Timing jitter is thedeviation of the measured photon arrival time from the actual photonarrival time. Reset time is the time it takes for the device to be resetto receive another photon after an initial photon is detected. Darkcount rate is the number of (false) detection signals produced per unittime when no photons are entering the device. For SPD's to be useful inthe widest possible applications they should have high signal-to-noise,low timing jitter, fast reset times, and low dark count rates.

There are numerous types of photon detector microelectronic circuitsknown in art. One type of SPD known in the art is an avalanchephotodiode (APD). APD's are highly sensitive semiconductor electronicdevices that exploit the photoelectric effect to convert light intoelectricity. An APD can be thought of as a photo-detector with abuilt-in first stage of gain through avalanche multiplication. From afunctional standpoint, they can be regarded as the semiconductor analogto photomultipliers. Over the past decade, superconducting nanowiresingle photon detectors (SN-SPDs) have become promising alternatives toconventional semiconductor avalanche photodiodes in the near-infraredregion of the spectrum. In particular, SN-SPDs based on niobium nitride(NbN) superconducting nanowires have demonstrated desirable properties30 picoseconds timing jitter, fast reset times on the order of 10nanoseconds, and low dark count rates below 1 kcps (thousand counts persecond), but generally suffer from low system detection efficiencies ofless than 20%. In addition, the efficiency of these detectors dependsstrongly on the polarization of the incident light.

Another problem known in the art is that the quantum efficiency ofSN-SPD's degrades as the width of the nanowires increase, and thereforemicroelectronic circuits in the prior art have been limited to the useof extremely narrow nanowires. The quantum efficiency and sensing areaof the microelectronic circuit are affected by the width of thenanowires and the width of channel spaces between the nanowires. NbNnanowires known in the art are arranged in a continuous meander patternacross the surface of the detector microelectronic circuit. In theory,larger geometric area covered by the nanowires should translate into aproportionately larger quantum efficiency of the microelectroniccircuit. However, nanowires frequently have width dimensions of theorder of the channels separating the nanowires from each other.Nanowires may have an approximate thickness of 5 nm and an approximatewidth of 100 nm. Current electron beam fabrication methods make itextremely difficult to create nanowires and channel features smallerthan 100 nanometers. A limiting factor governing the quantum efficiencyof SN-SPD microelectronic circuits based on NbN is that they have arelatively small area where absorption of photons may take place.

Another problem known in the art is that SPD devices experiencedecreased quantum efficiencies for photons having longer wave lengths.SPD devices currently known in the art, which utilize NbN, have achievedextremely high quantum efficiencies for photons whose wavelength is inthe UV, visible, and for some wavelengths in the near infrared region ofthe spectrum.

Another problem known in the art is that the detection efficiency ofSNSPD's depends upon the polarization of the light. There is an unmetneed for superconducting SPDs which can detect photons with close to100% efficiency for any photon wavelength, and any photon polarization.It is desirable to have SN-SPD technologies which provide larger areasfor the absorption of photons within a designated physical area of themicroelectronic circuit. It is desirable to have SN-SPD's with highsignal-to-noise.

SUMMARY OF THE INVENTION

The invention is a polarization independent superconducting nanowireavalanche photo-detector apparatus which has higher quantum efficiencyand signal-to-noise than any superconducting nanowire single photondetectors known in the art.

DETAILED DESCRIPTION OF INVENTION

For the purpose of promoting an understanding of the present invention,references are made in the text to exemplary embodiments of a singlephoton detector with optimized reactive surface geometry only some ofwhich are described herein. It should be understood that no limitationson the scope of the invention are intended by describing these exemplaryembodiments. One of ordinary skill in the art will readily appreciatethat alternate but functionally equivalent components may be used. Theinclusion of additional elements may be deemed readily apparent andobvious to one of ordinary skill in the art. Specific elements disclosedherein are not to be interpreted as limiting, but rather as a basis forthe claims and as a representative basis for teaching one of ordinaryskill in the art to employ the present invention.

It should be understood that the drawings are not necessarily to scale;instead emphasis has been placed upon illustrating the principles of theinvention. In addition, in the embodiments depicted herein, likereference numerals in the various drawings refer to identical or nearidentical structural elements.

Moreover, the terms “substantially” or “approximately” as used hereinmay be applied to modify any quantitative representation that couldpermissibly vary without resulting in a change in the basic function towhich it is related.

The present invention includes a three dimensional singlesuperconducting nanowire single photon detector (SN-SPD) microelectroniccircuit which has higher quantum efficiency than of SN-SPD's known inthe art. This is achieved because the unique material and configurationof the microelectronic circuit eliminates the polarization dependenceand shows improved signal-to-noise over SN-SPD microelectronic circuitsknown in the art. This objective is achieved by vertically stacking twotungsten-silicide (TS)-SN-SPDs made of amorphous tungsten silicide (TS)and electrically connecting them in parallel. This structure forms amultilayer superconducting nanowire avalanche photo-detector (SNAP). Asingle photon detection device employing the multilayer (SNAP)microcircuit demonstrates a peak system detection efficiency of 87.7%and a polarization dependence of less than 2%. This represents nearly anorder of magnitude improvement in both system detection efficiency andreduction of polarization dependence compared to conventional SNSPDs.

FIG. 1 is a cut-away perspective of an exemplary single layersuperconductive nanowire continuous meander pattern 100 using a tungstensilicide (TS) nanowire. FIG. 1 illustrates the prior art of a tungstensilicide (TS) nanowire sections 20 a through 20 n. The completegeometric pattern of the nanowire is referred to as a continuous meanderpattern (shown in FIG. 2).

In FIG. 1, the front right portion of the microelectronic circuit is cutaway to expose silicon substrate layer 10 and silicon oxide layer 12.Nanowire sections 20 a through 20 n are formed on an oxidized siliconsubstrate, which is a wafer of silicon on which a silicon oxide layer 12has been created using a process known in the art. The illustratedembodiment was prepared by co-sputtering tungsten and silicon to athickness of 4.5 nanometers. Then a poly(methylmethacrylate) resist wasapplied, and imaged using an electron beam. The areas revealed by theexposed resist are etched by reactive ion etching in sulfurhexafluoride. The area etched by reactive ion etching in the embodimentshown forms the etched channel spaces 25 a through n, and the areasprotected by the resist form the nanowire shown as nanowire sections 20a through 20 n. In the embodiment shown the nanowires have asuperconducting critical temperature of 3K, and an amorphous structure.In the exemplary embodiment the tungsten silicide (TS) nanowire sections20 a through 20 n are 150 nm wide and 4.5 nm thick. FIG. 1 illustratesetched channel spaces 25 a through 25 n which in the present embodimentis 100 nanometers wide.

In the embodiment shown, tungsten silicide (TS) nanowire sections 20 athrough 20 n are comprised of 25 mole percent silicon, and 75 molepercent tungsten (W_(0.75)Si_(0.25)). The use of a tungsten silicidenanowire overcomes the manufacturing yield limitations of devices basedon the conventional niobium nitride (NbN) superconductors because theamorphous tungsten silicide is less sensitive to defects than is thepolycrystalline niobium nitride.

FIG. 2 is a top perspective view of a superconductive nanowire detectormicroelectronic circuit 100 in the prior art illustrating the nanowirecontinuous meander pattern 30 and bonding pads 11 a, and 11 b. In theembodiment shown in FIG. 2 the nanowire continuous meander pattern 30has an area of 16 μm×16 μm.

Nanowire 20 and bonding pads 11 a, and 11 b, are fabricated withoutsignificant constriction in the wires that would suppress thesuperconductive critical current (I_(c)). The silicon substrate 10 alsocan be seen at the base of the etched channel spaces 25. In theexemplary embodiment shown the etched channel spaces 25 are 100 nm wide,and lie between the TS nanowires 20 a through 20 n. The coverage area ofthe TS nanowires 20 a through 20 n in the embodiment shown represents60% of the area of the continuous meander pattern 30. In the FIG. 2illustrates a nanowire continuous meander pattern 30 which in thisembodiment covers an area of 16 μm×16 μm. SN-SPD's based on an amorphoussuperconductor, tungsten silicide (TS), show that the use of thismaterial enables the fabrication of detectors with significantly widernanowire geometries (˜150 nm) and higher detection efficiencies (>90%)than NbN-based SNSPDs.

FIG. 2 also illustrates bonding pads 11 a, and 11 b allowing themicroscopic continuous meander pattern to electrically interface withelectronic testing equipment known in the art.

In order to enable the tungsten silicide to operate as a super conductorfor the detection of photons, certain requirements are necessary. Thetungsten silicide nanowires must be cooled to a temperature below butnear the superconductive critical temperature, and biased with a biascurrent less than the superconductive critical current. Tungstensilicide nanowires of the exemplary embodiment are superconductive attemperatures from 0 to 5K, dependent on the exact composition of thetungsten silicide alloy. At this temperature the nanowire has noresistance to the flow of current. The absorption of a photon underthese conditions is believed to create a “hot spot” in the TS nanowire20.

FIG. 3 is a side sectional perspective view of an exemplary embodimentof a multilayer superconducting nanowire avalanche photo-detector (SNAP)microelectronic circuit 400. The exemplary embodiment showed in FIG. 3illustrates a device architecture in which multiple nanowire continuousmeander pattern sections are vertically stacked on top of each other.This architecture is referred as a multilayer SNAP. FIG. 3 illustrates atop continuous meander pattern 44 a and a bottom continuous meanderpattern 44 b separated by a dielectric layer 42, and a silicon substrate10. FIG. 3 also illustrates left bonding pad 30 a, and right bonding pad30 b. The arrangement allows the top continuous meander pattern 44 a,and the bottom continuous meander pattern to be electrically connectedin parallel.

The multilayer superconducting nanowire avalanche photo-detector (SNAP)illustrated in FIG. 3 is a continuous nanowire meander pattern with Nparallel sections, the bias current (I_(B)) must be increased beyond theavalanche threshold current I_(AV) to ensure correct operation. Uponabsorption of a photon one out of N sections switches to the normalstate, diverting its current to the remaining N−1 sections and drivingthem into the normal state. Thus, an electrical current N times higherthan the current of a single section is diverted to the load resistor,and the signal-to-noise ratio is improved by a factor of N. Nanowiredetectors utilizing this parallel architecture with N sections havebecome known as cascade-switching super-conducting single-photondetectors or superconducting nanowire avalanche photo-detectors(N-SNAPs).

The exemplary embodiment illustrated in FIG. 3 shows vertically stackedthe sections of SNAP's on top of each other. This architecture isreferred to as a multilayer (SNAP). Furthermore, because the twosections could be patterned independently, the two sets of nanowiresshown in FIG. 3 are oriented at orthogonal angles with respect to oneanother. The vertical stacking of orthogonal nanowire continuous meanderpatterns connected electrically in parallel has produced (1) a factor of˜2 higher signal-to-noise ratio than previously reported with TS SNSPDs;(2) polarization independent system detection efficiency (SDE) over a˜100 nm-wide wavelength range; and (3) system detection efficienciesgreater than 85%, comparable to the best results achieved to date withplanar SNSPDs.

Each section of the multilayer SNAP consists of a 16 micrometer×16micrometer nanowire continuous meander pattern with a nanowire width of150 nm and a pitch of 350 nm. The thickness of each TS layer is 4.5 nm,yielding a superconducting critical temperature (T_(c)) of approximately3 K, slightly lower than the bulk T_(c) of 5 K for tungsten silicidealloy with an optimized Si composition of approximately 25%. These twonanowire meanders are separated by a 30 micrometer×30 micrometer, 75nanometer thick square pad of hydrogen silsesquioxane (HSQ), anegative-tone pattern-able electron beam resist that becomes amorphoussilicon oxide after exposure to an electron beam. The amorphous siliconoxide layer serves as a dielectric (electrical insulator) between thetwo SNSPDs. The top continuous meander pattern 44 a and the bottomcontinuous meander pattern 44 b are in contact with each other where theHSQ has been developed away on each side of the patterned silicon oxidesquare pad, so that the two sections are electrically connected inparallel.

While the deposition of a crystalline superconductor such as NbNdirectly on top of an amorphous material would be impossible withoutdegradation of its superconducting properties, the amorphous nature ofamorphous TS allows it to be deposited on silicon oxide without anydegradation of the T_(c) or the superconducting switching current(I_(SW)) of the top layer compared to the bottom layer. The quality ofboth layers was tested by characterizing the superconducting propertiesof single detectors both on top of and underneath the silicon dioxidedielectric layer.

Several layers not shown in FIG. 3 are added to increase the detectionefficiency. The multilayer SNAP was embedded in a stack of dielectricmaterials to optimize absorption at a wavelength of 1550 nm. The stackis designed such that the antinode of the electric field was positionedhalfway between the two TS detector layers, so that each detector layerabsorbs an equal number of photons. The stack consists of an aluminummirror and λ/4 layer of SiO₂ below the detector and four alternatinglayers of SiO₂ and silicon nitride above the detector. After thedeposition of the top layers of the optical stack, a keyhole-shape wasetched around the detector through the silicon substrate, and theresulting microelectronic circuit was removed, packaged, and aligned toa standard telecommunications single-mode optical fiber by use of aself-aligned packaging scheme.20 This simplified packaging schemeensures excellent alignment (63 lm) between the detector and the opticalfiber.

FIG. 4 a is a scanning electron micrograph of an exemplary embodiment ofa multilayer SNAP microelectronic circuit 400. FIG. 4 shows the topcontinuous meander pattern 44 a as vertical lines and the bottomcontinuous meander pattern 44 b as lighter horizontal lines. The bottomcontinuous meander pattern is lighter because it is partially obscuredby dielectric layer 42. FIG. 4 a also shows left bonding pad 30 a, andright bonding pad 30 b.

FIG. 4 b is a higher magnification scanning electron micrograph of anexemplary embodiment of a multilayer SNAP microelectronic circuit 400.FIG. 4 shows the top continuous meander pattern 44 a as vertical linesand the bottom meander pattern 44 b as lighter horizontal lines. Thebottom continuous meander pattern is lighter because it is partiallyobscured by dielectric layer 42.

FIG. 5 is a wiring diagram of how an exemplary embodiment of amultilayer SNAP microelectronic circuit is wired. FIG. 5 illustrates apower supply 450, a bias current 455, a resistor 460, top continuousmeander pattern inductor element 470, bottom continuous meander patterninductor element 480, and large inductor 490. Large inductor 490 iselectrically connected in series with top continuous meander patterninductor element 470, and bottom continuous meander pattern inductorelement 480. The large inductor 490 is fabricated on-microelectroniccircuit in the top layer of tungsten silicide away from the detectorarea and is not illustrated in FIG. 3.

FIG. 6 a is a top perspective view of an exemplary embodiment of amultilayer SNAP device 200. FIG. 6 a shows optical fiber 50, coaxialconnectors 90, and thermal housing 120. In the exemplary embodimentshown the detector is mounted inside an adiabatic demagnetizationrefrigerator and cooled to ˜150 milliK for measurements of the SDE(defined as the probability of detecting a photon that has been coupledinto the fiber) and dark count rate (DCR).

FIG. 6 b is a diagram of the optical components of an exemplaryembodiment of a multilayer SNAP test system. FIG. 6 b shows an exemplaryembodiment of a multilayer SNAP test device 200. FIG. 6 b shows tunablelaser 500 which in the exemplary embodiment illustrated is a 1 mW lasertunable from (1520 nm to 1630 nm) through a computer controlledpolarization controller. FIG. 6 b illustrates optical attenuators 520,set to provide a count rate of approximately 50,000 photons/sec. FIG. 6b shows micro electro-mechanical system (MEMS) optical switch 530 usedto switch between a power meter and the device under test. Before eachmeasurement, the MEMS switch output was sent to the power meter forcalibration of the number of photons per second incident on thedetector, then switched to the device under test for measurement of theSDE.

FIG. 7 a is a plot showing the system detection efficiency (SDE) of anexemplary embodiment of a multilayer SNAP device as a function of biascurrent. In the exemplary embodiment shown in FIG. 7 a light of 1550nanometers is used. An SDE of 85.7%±0.6% is measured for the exemplaryembodiment shown.

FIG. 7 b is a plot showing the dark count rate (DCR) of an exemplaryembodiment of a multilayer SNAP device. Conditions used for themeasurements shown in FIG. 7 b are the same as those for themeasurements shown in FIG. 7 a. For the exemplary embodiment shown theDCR is lower than 400 cps with a fiber coupled to the detector, which weattribute to stray blackbody radiation coupled into the fiber. Without afiber coupled to the detector, the device exhibits a DCR of less than0.1 cps. For the two detectors connected in parallel the switchingcurrent of 9.8 microamperes is approximately twice the switching currentof a typical single-layer SN-SPD. From the bias dependence of the SDE,we find an avalanche current I_(AV) ˜0.6×I_(SW). The device exhibits abroad plateau in SDE over 40% of the bias range between the avalanchecurrent and the switching current.

Although the larger signal-to-noise ratio is a benefit of the SNAParchitecture, stacking the two sections of the multilayer SNAP atorthogonal angles eliminates the polarization dependence of the SDE inthe exemplary embodiment.

FIG. 8 a is a map of the single-photon detection efficiency (SDE) overthe Poincaré sphere of an exemplary embodiment of a multilayer SNAPdevice. The SDE values at 1560 nm are shown in a color scale andindicate the variation in efficiency over the whole range of possiblepolarizations. In FIG. 8 a the horizontal and vertical axes representthe axial ratio (AR) where AR=(major axis)/(minor axis), ∈=cot(AR), andthe tilt angle (θ) of the polarization ellipse of the electric fieldvector of the photon. Using this notation, the quantities 2∈ and 2θrepresent latitude and longitude on the Poincaré sphere. The entirePoincaré sphere is mapped by −90°<2∈<90° and 0<2θ<360°.

FIG. 8 b is a map of the single-photon detection efficiency (SDE) overthe Poincaré sphere of a standard superconducting nanowire single photondetector (SN-SPD) device. The standard single-layer SNSPD exemplifyingthe prior art is a single-layer SNSPD had a wire width of 120 nanometersand a pitch of 220 nanometers. The device was embedded in an opticalstack consisting of (bottom to top) a gold mirror, a layer of sputteredSiO₂, the tungsten silicide detector, a second layer of SiO₂, and asputtered layer of TiO₂, with layer thicknesses optimized for absorptionat 1550 nm. Note the difference in scales.

Comparing FIG. 8 a of multilayer SNAP device and FIG. 8 a of the SN-SPDdevice the multilayer SNAP device shows a variation of less than 2%,over the Poincaré sphere compared to the single-layer device, whichshows a variation of approximately 16%.

FIG. 9 a is a plot of the maximum and minimum SDE over the Poincarésphere of an exemplary embodiment of a multilayer SNAP device. The SDEreaches a peak of 87.7%±0.5% at 1540 nm, close to the design wavelengthof 1550 nm. At shorter and longer wavelengths, the SDE decreases due tothe effects of the optical stack.

FIG. 9 b is a plot of the ratio of the maximum to minimum SDE(SDE_(max)/SDE_(min)) as a function of wavelength for anIndium-Gallium-Arsenide (InGaAs) power meter and for an exemplaryembodiment of a multilayer SNAP device. The ratioR_(DSE)=(SDE_(max)/SDE_(min)) for the multilayer SNAP device (indicatedby solid boxes) shows a minimum of 1.019 at 1560 nm and remains below1.04 over the entire wavelength span from 1510 nm to 1630 nm. The factthat R_(sDE) never reaches unity is attributed to the polarizationdependence of the optical components of the measurement setup. Tosupport this contention FIG. 9 b also shows the wavelength dependence ofthe ratio between the maximum and minimum power measured by an InGaAspower meter while scanning the polarization over the Poincaré sphere,indicated by the blue triangles in FIG. 9 b. The similarity in thepolarization dependence of the components of the measurement setup(shown in FIG. 6 b) is indicated by the overlap of the triangles overlapwith the squares within the error bars of the measurement over awavelength range from 1520 nm to 1580 nm, indicating that the primarysource of the small polarization dependence is the measurement setup andsuggesting that the multilayer SNAP itself is polarization independent.

For applications requiring high repetition rates, the rise time, decaytime, and dead time of the detector are important metrics. In theexemplary embodiment a bias current of 9 microamperes an amplifiedvoltage pulse height of 600 millivolts is measured with a rise time of 8nanoseconds, a 1/e decay time of 57 nanoseconds, and a dead time (duringwhich the system detection efficiency is suppressed) of 38 nanosecondsas estimated from inter-arrival time measurements. The long rise anddecay times are due to the large series inductor (490 in FIG. 5) beingten times the inductance of continuous meander pattern inductors (470and 480 in FIG. 5), required to ensure stable operation of the SNAP. Asa result of the long rise time, we measure a relatively large jitter of˜465 picoseconds.

Although the measured jitter is large relative to that of typicalSNSPDs, it is significantly smaller than that of other single photondetectors such as transition edge sensors (TESs), and comparable to thejitter of commercially available Si APDs. The jitter of 465 picosecondsis compatible with the measurement of multi-photon entangled statesproduced at a repetition rate of 80 MHz by a Ti:Sapphire mode-lockedlaser. The mulitlayerSNAP would also provide for a smaller coincidencewindow than a TES in a loophole-free test of the Bell inequalityallowing a smaller distance between detectors and improvement in thesystem detection efficiency in such a measurement. The jitter of themulitlayerSNAP may be reduced by reducing the value of the largeinductor 490 in FIG. 5, which can be done by reducing the inductance ofeach individual nanowire continuous meander patterns 470 and 480 in FIG.5. This can be accomplished using a “nested” SNAP geometry, in which thetop and bottom continuous meander patterns are themselves SNAPs.

The extension of the SNAP nanowire architecture into three-dimensionsrepresents a significant advancement in single-photon detectortechnology. Improvements in optical stack design and fabrication and theuse of a higher-fill-factor nanowire continuous meander pattern shouldallow for an SDE approaching 100%. The significant reduction of thepolarization dependence of the SDE will enhance the overall detectionefficiency in experiments where the light is un-polarized and willeliminate the need for polarization controllers and wave-plates inexperiments where the light is strongly polarized. Finally, stackingmore than two layers may provide a route to obtaining detectionefficiencies approaching 100% that are less wavelength dependent, whichmay be important for applications requiring high detection efficiencyover a broad range of wavelengths.

What is claimed is:
 1. A microcircuit apparatus for detecting single photons without regard to polarity comprised of: a base layer; a first superconducting layer having a first superconducting nanowire patterned to form a first continuous meander pattern; wherein said first continuous meander pattern is comprised of parallel line segments joined by curved segments; a dielectric layer comprised of material wherein said dialectic layer is substantially transparent to a predetermined photon wavelength; and a second superconducting layer having a second superconducting nanowire patterned to form a second continuous meander pattern wherein said second continuous meander pattern is comprised of parallel line segments joined by curved segments;
 2. The apparatus of claim 1 wherein substantially all of said parallel line segments of said first continuous meander pattern are orthogonally oriented to said parallel line segments of said second continuous meander pattern.
 3. The apparatus of claim 1 wherein said first superconducting nanowire and said second superconducting nanowire are comprised of an amorphous metal-metalloid alloy.
 4. The apparatus of claim 3 wherein said amorphous metal-metalloid alloy is comprised of an alloy of tungsten and silicon.
 5. The apparatus of claim 4 wherein said alloy of tungsten and silicon is comprised of 20 mole percent to 30 mole percent silicon.
 6. The apparatus of claim 1 wherein said dielectric layer is comprised of a material selected from a group consisting of an oxide of silicon and a nitride of silicon.
 7. The apparatus of claim 1 wherein said dielectric layer is comprised of a transparent material selected from a group consisting of an oxide, a nitride and a fluoride.
 8. The apparatus of claim 1 wherein said dielectric layer is comprised of a semiconductor selected from a group consisting of silicon or germanium.
 9. The apparatus of claim 1 wherein said predetermined photon wavelength is approximately 300 nanometers to 3,000 nanometers.
 10. The apparatus of claim 1 wherein said predetermined photon wavelength is approximately between 800 nanometers and 2000 nanometers.
 11. The apparatus of claim 1 wherein said predetermined photon wavelength is approximately 1050 nanometers and 1600 nanometers.
 12. The apparatus of claim 1 wherein said predetermined photon wavelength is approximately 1800 nanometers and 2400 nanometers.
 13. The apparatus of claim 1 wherein said predetermined photon wavelength is approximately between 2000 nanometers and 8000 nanometers.
 14. The apparatus of claim 1 wherein said first superconducting nanowire layer and said second superconducting nanowire layer are connected in series to electrical inductor components having an inductance greater than the inductance of said first superconducting nanowire layer and said second superconducting nanowire layer.
 15. The apparatus of claim 14 wherein said inductance of said inductor component is at least 8 times the inductance of said first superconducting nanowire layer and said second superconducting nanowire layer.
 16. The apparatus of claim 1 which further includes one or more dielectric layers on said second superconducting nanowire layer.
 17. The apparatus of claim 1 in which said base layer is deposited on one or more alternating layers of silicon oxide, and silicon nitride.
 18. The apparatus of claim 17 in which said one or more alternating layers of silicon oxide, and silicon nitride are deposited on a mirror layer consisting of a metal.
 19. The apparatus of claim 1 wherein the width and pitch of said continuous meander pattern of said first superconducting nanowire layer are substantially equivalent to the width and pitch of said continuous meander pattern of said second superconducting nanowire.
 20. The apparatus of claim 1 wherein the width and pitch of said continuous nanowire meander pattern of said first superconducting nanowire layer are not equivalent to the width and pitch of said continuous meander pattern of said second superconducting nanowire.
 21. The apparatus of claim 1 wherein the width of said first superconducting nanowire and said second superconducting nanowires is between 3 nanometers and 3000 nanometers.
 22. The apparatus of claim 1 wherein the thickness of said dielectric layer is between 3 nanometers and 3000 nanometers.
 23. The apparatus of claim 1 wherein a detection area produced by said first and said second continuous meander pattern is at least 125 square micrometers.
 24. A method of making a microcircuit apparatus for detecting single photons without regard to polarity comprised of: forming a base layer; forming first superconducting layer on said base layer; patterning said first superconducting layer to form a first continuous meander pattern on said base layer wherein said continuous meander pattern is comprised of a series of substantially parallel line segments joined by curved segments to form said continuous meander pattern; selecting a dielectric material to correspond to a predetermined photon wavelength; forming a dielectric layer comprised of said dielectric material wherein said dialectic layer is substantially transparent to a predetermined photon wavelength; and maintaining said microcircuit apparatus in a chamber that has a temperature of below 5 Kelvin.
 25. The method of claim 26 which further includes the step of forming a second superconducting layer on said dielectric layer.
 26. The method of claim of 26 which further includes the step of patterning said second superconducting layer with a continuous meander pattern orthogonally oriented to said first continuous meander pattern. 